Invalid CSRF Token ‘null’ was found on the request parameter ‘_csrf’ or header ‘X-CSRF-TOKEN’

It looks like the CSRF (Cross Site Request Forgery) protection in your Spring application is enabled. Actually it is enabled by default.

According to spring.io:

When should you use CSRF protection? Our recommendation is to use CSRF
protection for any request that could be processed by a browser by
normal users. If you are only creating a service that is used by
non-browser clients, you will likely want to disable CSRF protection.

So to disable it:

@Configuration
public class RestSecurityConfig extends WebSecurityConfigurerAdapter {
  @Override
  protected void configure(HttpSecurity http) throws Exception {
    http.csrf().disable();
  }
}

If you want though to keep CSRF protection enabled then you have to include in your form the csrftoken. You can do it like this:

<form .... >
  ....other fields here....
  <input type="hidden"  name="${_csrf.parameterName}"   value="${_csrf.token}"/>
</form>

You can even include the CSRF token in the form’s action:

<form action="./upload?${_csrf.parameterName}=${_csrf.token}" method="post" enctype="multipart/form-data">